The infrared transmission spectrum of Si-doped molecular beam epitaxy (MBE)-grown GaAs epilayers, $2–2.5-\mu \mathrm{m}$ thick, measured in the oblique (Berreman) geometry, revealed distinct minima in p polarization. Given epilayer thickness $\ll$ reststrahlen wavelength, the minima are identified as the zone-center transverse optic phonon $\left({\omega }_{\mathrm{TO}}\right)$ and the high-frequency LO phonon-plasmon coupled mode $\left({\omega }_{+}\right).\mathrm{}$ Analysis of the experimental data yielded free-carrier concentrations ranging from $2.5\times {10}^{17}$ to $1.4\times {10}^{18}{\mathrm{cm}}^{-3}.$ The same technique with MBE-grown Si-doped ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ epilayers $(0.5\mathrm{}–1\mu \mathrm{m}$ thick) yielded ${\omega }_{+}$ modes corresponding to free-carrier concentrations of $8.2\times {10}^{16}–2.7\times {10}^{19}{\mathrm{cm}}^{-3}.$ The observations of the transmission minima in the Berreman geometry and their interpretation demonstrate a direct and simple method for deducing free-carrier concentrations over a wide range.

• Received 20 March 2003

DOI:https://doi.org/10.1103/PhysRevB.69.075314